There are DC converters with galvanic decoupling between input DC voltage and output DC voltage, and there are DC converters without such decoupling. An example of a known DC converter in which galvanic decoupling is effected with the aid of a transformer is shown in FIG. 10 on page 10/17 of the data specification VIPer 100/SP-VIPer 100/A/ASP of SGS-THOMSON MICROELECTRONICS from 1997 which is incorporated herein by reference. The transformer of this known DC converter has two primary windings. One is connected with an electronic switch in the form of a MOS transistor for chopping the input DC voltage available across an intermediate circuit capacitor to make it transformable into another voltage. The MOS transistor is part of a monolithic integrated circuit, called a VIPer, which contains a pulse-width modulation circuit including a loop and oscillator for controlling the turn-on and -off times of the MOS transistor. The VIPer integrated circuit is fed from a second primary winding of the transformer and performs pulse-width modulation of the pulses, switching the MOS transistor in accordance with the charging voltage of a capacitor charred via the second primary winding by comparing its charging voltage with a reference voltage. A more detailed view of the VIPer integrated circuit presented very schematically in FIG. 10 of the stated print which is incorporated herein by reference is found in the block diagram on page 1/17 of said print.
Such a transformer with two primary windings is expensive and elaborate. For applications not requiring galvanic decoupling between the input DC voltage and the output DC voltage, DC converters have become known which work without a transformer. A known DC converter of this kind has a series connection comprising a resistor, a first capacitor and a zener diode which are connected between the two input terminals of the DC converter. In parallel with the zener diode there is a series connection comprising a diode and a second capacitor. The charging voltage of the second capacitor serves as the output DC voltage. The two capacitors are charged via the resistor and the diode until the charging voltage of the second capacitor has reached the zener breakdown voltage of the zener diode. The output DC voltage thus depends on the zener voltage of the zener diode. In this circuit the power not taken by the load on the output side is dissipated in the zener diode. Even if little power is taken on the consumer, there is thus always a power dissipation to the amount of the nominal power. Altogether the power dissipation is lower than in series resistor power packs, however. But the first capacitor must be relatively large, which causes high costs for such a DC converter.
Another known DC converter without galvanic decoupling has between the two input terminals a series connection comprising a diode, a resistor and a zener diode, the zener diode having connected in parallel therewith a capacitor from which the output DC voltage can be taken. The disadvantage of this DC converter is its high power dissipation.
These two DC converters without galvanic decoupling are furthermore only suitable for outputs in the range up to about 3 to 4 W.
A further known DC converter without galvanic decoupling has a series connection comprising a MOS switching transistor and a choke in one series arm between input terminals and output terminals. A voltage divider is optionally connected between the output terminals, its component voltage being fed as the control variable to a pulse-width modulation circuit which provides pulse-width modulated switching pulses to the MOS switching transistor.
This known DC converter circuit is limited today to input DC voltages of about 50 V for monolithic integrated total circuits.